1. Field of the Invention
This invention relates to an apparatus and method for calibrating an electromagnetic logging tool such as an induction logger, that is used to measure the conductivity of the surrounding formation while a borehole is being drilled therethrough. The apparatus is particularly useful as a secondary standard for field use in testing MWD loggers.
2. Discussion of the Prior Art
Electromagnetic (induction) logging tools for measuring the electrical properties of subsurface earth formations are well known. The logging tool may be run into a borehole on a wireline after the hole has been drilled. Alternatively the tool may be incorporated into the lower end of the drill string, near the bit so as to log the formation parameters while the borehole is being drilled (i.e. Measurement While Drilling or MWD). The advantage of the latter method is that he tool can measure formation parameters before the borehole sidewall has become contaminated with the drilling fluids. Furthermore, the desired measurements are gathered in near real time rather than days or weeks later as is the case with wireline logging.
Typical induction tools consist essentially of a transmitter coil and two receiver coils that are mounted concentrically on a mandrel. The two receiver coils may be separated by, perhaps, 30 cm and the transmitter coil may be about 0.7 m from the nearest receiver coil. In some tools, a second transmitter is provided at the opposite end of the receiver array. The respective coils commonly consist of several turns of wire of a suitable size that are insulated from the mandrel. An electronics module is incorporated into the tool to provide power to the transmitter coil and to detect the signals in the receiver coils. Means are furnished to either store the measured formation parameters or to transmit the gathered data to surface equipment, or both.
At timed intervals such as every 100 milliseconds (ms), a 1-or 2-mHz sinusoidal signal at a potential of about 200 volts is launched into the transmitter coil. If two transmitter coils are provided, the transmitters are excited in alternative cycles and the received signals resulting alternate cycles are averaged. The electromagnetic field radiates outwardly from the transmitter, through the formation surrounding the borehole, to each of the receivers in turn. The received signals are very weak, on the order of 1 to 500 .mu.v. The phase difference between the signals received by the respective receivers, as well as the amplitude ratio are parameters that are a function of the formation resistivity which may range from about 0.1 to about 100 ohm-meters or more. Those measurements provide a penetration depth into the sidewall formation of nearly two meters.
An induction logging tool is essentially a precision voltmeter that must be calibrated with respect to a laboratory standard. Usually, the tool is calibrated in a large brine tank where the resistivity of the test medium (salt water) can be accurately controlled. The test results take the form of response curves plotted as formation resistivity vs. phase shift and amplitude ratio. See for example, FIGS. 1 and 2.
MWD logging tools must be rugged to withstand the drilling stresses. Made of conductive but non-magnetic metal such as beryllium-copper or monel metal, an MWD logging tool may be two or three meters long, 15 in diameter and is quite heavy. A secondary standard is used for field calibration of an MWD induction logging tool in the field both before and after a logging run. Of course, there is no reason why a portable secondary standard could not also be used on-site to confirm the calibration of a wireline tool.
The theory of the operation of an inductive logging tool mounted on a conductive mandrel is explained in a paper entitled The theory of 2 mHz Resistivity Tool and its Application to Measurement-While-Drilling by D. Coope et al. in The Log Analyst, v. 25, No. 3, May-June, 1984. The concepts are well known. Hence it is not considered to be necessary here to expand further upon the theory of operation. An analysis of the response of an MWD tool is presented by Q. Zhou et al. in MWD Resistivity Tool Response in a Layered Medium, published in Geophysics, v. 56, No. 11, November 1991.
U.S. Pat. No. 4,876,511, issued Oct. 24, 1989 to Brian Clark for a Method and Apparatus for Testing and Calibrating an Electromagnetic Logging Tool, discloses an external testing apparatus for a logging tool which includes at least a transmitting antenna and dual-channel receiving antennas. A shielded receiving device is clamped around the transmitting antenna and intercepts the transmitted signal. A shielded transmitting device is positioned around the receiving antennas of the tool and transmits to the receiving antennas a signal which has a phase and/or amplitude that is related to the signal transmitted by the tool's transmitting antenna in a known manner. It simulates the effect that a geological formation would have on the signal if it were to travel from the tool's transmitting antenna through the formation. Since the simulated effect is known, the output of the tool may be verified as being correct or it may be corrected if it is erroneous.
Another reference is found in U.S. Pat. No. 5,001,675, issued Mar. 19, 1991 to G. H. Woodward for a Phase and Amplitude Calibration System for Electromagnetic Propagation Based on Earth Formation Evaluation Instruments. The invertor teaches an automatic calibration system which compensates for errors caused by temperature and pressure variations in a borehole. In this system, an auxiliary calibration antenna is tightly coupled physically to each of the spaced-apart receiving antennas of a dual-channel logging tool. In-phase equal-amplitude reference signals are applied to the two measurement channels from the calibration antennas to the respective receiving antennas. Each of the calibration antennas communicates with an attenuator which acts to make the calibration antennas transparent to the receiving antenna when the latter are in the measurement mode.
A Russian paper entitled Calculation of Calibration Loops for Induction Logging Tools, by V. N. Boganik et al., published in News From Colleges, 1965, No. 9, describes a single closed calibration loop that may be used with a wireline tool.
The first reference discloses a rather awkward calibration assembly that must be clamped around the logging tool mandrel. In addition, a good deal of complex circuitry is required to match the calibration antennas to the receiving and transmitting antennas that are mounted on the tool itself. The second reference is merely a relative compensation device to equalize the parameter measurements from different depths to a common base line. The device of the '675 patent is not an absolute-value calibration tool. The Russian device is a single closed loop suitable for use with a wireline logger but not with MWD equipment.
There is a need for a simple, portable secondary calibration standard that first can be exercised in the laboratory and later used for field testing of a logging tool. It is desirable that the calibration device should be passive and not dependent for its operation upon auxiliary circuitry which, itself, would require calibration.